Class vehicle with virtual function

**silicon** · 12-22-2003

Hi everyone, I wrote a rough draft of a code that is supposed return the type of car you are driving. I kind of got the hang of it but wanted to go ahead in the book that I am reading and implement a virtual function. I read about them but was not sure how I could implement them. I wanted to have it so that after the program looked at all of the data entered..it would then printout the sound that the car was supposed to make (horn function) sportscar would make a "beep" and suv would make a "BEEP"..something like that.
Am i correct in the fact that virtual function is a function that can be accessed and changed by the entire code???
Is what I am trying to do possible or do I need to start over?

Any help would greatly be appreciated
Thanks in advance..especially to SirCrono6, laasunde, and Zach L. for previous help

Code:

#include <iostream.h>


class Vehicle
{
  public:

    Suv();
Sportscar();
virtual horn ();
  private:
    int doors;
    int cylinders;
	int name;
	int color;
	char transmission;
	int a;
	int m;
};


Vehicle::Suv()
{
 cout<<" How many doors does the car have? ";
 cin>>doors;
cout<<" How many cylinders does the car have? ";
cin>>cylinders;
cout<< " What kind of transmission does the car have? ";
cin>> transmission;	
 if (doors>=5 && cylinders>=6 )
	 cout<<" You are driving an SUV"<<endl;
	else
		cout<<" You are driving a sportscar"<<endl;
	if (transmission=='a')
		cout<<"with automatic transmission"<<endl;
	else
		cout<<"with manual transmission"<<endl;
	
 
}

Vehicle::Sportscar()
{
cout<<" How many doors does the second car have? ";
cin>>doors;
cout<<" How many cylinders does the second car have? ";
cin>>cylinders;
cout<< " What kind of transmission does the second car have? ";
cin>> transmission;	
	
	if (doors>=4 && cylinders>=6 )
	 cout<<" Your second car is an SUV"<<endl;
	else
		cout<<" Your second car is a sportscar"<<endl;
	if (transmission=='a')
		cout<<"with automatic transmission<<endl";
	else
		cout<<"with manual transmission"<<endl;
}

int main()
{
	Vehicle c;
	
	c.Suv();
	c.Sportscar();

return 0;
}

**Zach L.** · 12-22-2003

Virtual functions actually work a bit differently. They deal with when you have classes deriving from one another. Basically, if you have a pointer or a reference to a base class, that actually points to a derived class, then calling a virtual function f() will call the function belonging to the derived class. If it is not virtual, then the base class's method will be called.

Here is a short (and compilable) example that should help:

Code:

#include <iostream>

class A
{
public:
  virtual void f()
  {
    std::cout << "f() called from A" << std::endl;
  }

  void g()
  {
    std::cout << "g() called from A" << std::endl;
  }
};

class B : public A
{
public:
  void f()
  {
    std::cout << "f() called from B" << std::endl;
  }

  void g()
  {
    std::cout << "g() called from B" << std::endl;
  }
};

int main()
{
  B obj;
  A& ref = obj; // Base class reference pointing to an object of the derived class
  ref.f(); // Calls B::f() since f() is virtual
  ref.g(); // Calls A::g() since g() is not virtual
  std::cin.get();
}

So, Suv and Sportscar need to be classes that derived for vehicle, both implementing their own version of horn().

**silicon** · 12-23-2003

Hello, I believe that I am getting close. I just wanted to make sure that I was on the right track. I wanted to implement 2 functions for the two types of cars (suv BEEP and sportscar beep)

Code:

#include <iostream.h>


class Vehicle
{
  public:

    Suv();
	Sportscar();
	 virtual void horn()
  {
 cout << "The sound of your horn is beep" << endl;
  }

  void g()
  {
 cout << "The sound of your horn is BEEP" << endl;
  }
  private:
    int doors;
    int cylinders;
	int name;
	int color;
	char transmission;
	int a;
	int m;
};



Vehicle::Suv()
{
 cout<<" How many doors does the car have? ";
 cin>>doors;
cout<<" How many cylinders does the car have? ";
cin>>cylinders;
cout<< " What kind of transmission does the car have? ";
cin>> transmission;	
 if (doors>=5 && cylinders>=6 )
	 cout<<" You are driving an SUV"<<endl;
	else
		cout<<" You are driving a sportscar"<<endl;
	if (transmission=='a')
		cout<<"with automatic transmission"<<endl;
	else
		cout<<"with manual transmission"<<endl;
	
 
}

Vehicle::Sportscar()
{
cout<<" How many doors does the second car have? ";
cin>>doors;
cout<<" How many cylinders does the second car have? ";
cin>>cylinders;
cout<< " What kind of transmission does the second car have? ";
cin>> transmission;	
	
	if (doors>=4 && cylinders>=6 )
	 cout<<" Your second car is an SUV"<<endl;
	else
		cout<<" Your second car is a sportscar"<<endl;
	if (transmission=='a')
		cout<<"with automatic transmission"<<endl;
	else
		cout<<"with manual transmission"<<endl;
}

int main()
{
	Vehicle c;
	c.Suv();
	c.Sportscar();
	c.g();

return 0;
}

**jlou** · 12-23-2003

Sorry to have to tell you this, but you aren't really close, and you aren't on the right track. I'm not sure if you looked at Zach L.'s response. If you did, you must not have understood it, because I don't see where you implemented any of his suggestions. He showed two classes, one derived from the other. He also suggested that you make three classes, Vehicle, SUV, and Sportscar. SUV and Sportscar would derive from Vehicle in the same way that B is deriving from A in his example. Does that make sense?

**silicon** · 12-23-2003

Hello, I believe that now I am getting a little bit closer.

Thanks for the input so far everyone

Code:

#include <iostream.h>


class Vehicle
{
  public:

    Suv();
	Sportscar();
	 virtual void horn()
  {
 cout << "The sound of your horn is beep" << endl;
  }

  void g()
  {
 cout << "The sound of your horn is BEEP" << endl;
  }
  private:
    int doors;
    int cylinders;
	int name;
	int color;
	char transmission;
	int a;
	int m;
};



Vehicle::Suv()
{


	 cout<<" You are driving an SUV"<<endl;
	
}

Vehicle::Sportscar()
{


	 cout<<" You are driving a sportscar"<<endl;
	
}

int main()
{
Vehicle c;
int door;
int cylinders;
int transmission;
cout<<" How many doors does your car have";
cin>>door;
cout<<" How many cylinders does the second car have? ";
cin>>cylinders;

 if (door>=5 && cylinders>=6 )
c.Suv(),c.g() ;

 else
	 c.Sportscar(),c.horn();
 
	

 cout<< " What kind of transmission does the second car have? ";
cin>> transmission;	

	

	

return 0;
}

**Omnius** · 12-23-2003

You're still missing the point I'm afraid. I think you need to go back to your C++ book for an hour or two and familiarise yourself with inheritance - base classes and derived classes. Then virtual functions will become more meaningful to you. You can then come back to the example provided by Zach L and it will help you to understand how to use virtual functions. You can then try to apply it to your vehicle program.

**Zach L.** · 12-23-2003

The big thing you are still missing is that you need three separate classes to get your virtual functions to behave as you want them. 'Vehicle' should be your base class with the virtual 'horn' function. 'Suv ' and 'Sportscar' need to b ederived from the 'Vehicle' class, and then implement their own version of 'horn' (with the same function signature as the original).

As Omnius suggested, it would be advisable to go back to your C++ book and look over the inheritance chapter to familiarize yourself with it a bit more.

**silicon** · 12-23-2003

Hello everyone, I went back to the book and I believe that I have fixed some of my problems. This was a little bit more complicated than I thought. I implemented some of your suggestions to give myself a rough draft to work with. If it is still wrong I will work on this for a couple of days before I post more code.

Thanks everyone for being patient with me

Code:

#include <iostream.h>

class Vehicle
{
public:
	
  virtual void horn()
  {
   cout << "Horn" << endl;
  }

};

class Suv : public Vehicle
{
public:
  void horn()
  {
    cout << "You cars horn goes BEEP and you are driving an SUV" << endl;
  }

  void b()
  {
    cout << "Your car is black with automatic transmission" << endl;
  }
};

class Sportscar : public Vehicle
{
public:
  void horn()
  {
    cout << "Your car's horn goes beep and you are driving a sportscar" << endl;
  }

  void r()
  {
    cout << "Your car is red with manual transmission" << endl;
  }
};

int main()
{

	int door;
	int cylinders;
  cout<<" How many doors does your car have";
cin>>door;
cout<<" How many cylinders does your car have? ";
cin>>cylinders;
Suv v;  
Sportscar c;
  Vehicle& ref = v;// Base class reference pointing to an object of the derived class
  Vehicle& refer = c;

 if (door>=5 && cylinders>=6 )
  
  ref.horn(),v.b(); // Calls Suv::horn() since f() is virtual
 else
  refer.horn(),c.r();
 
}

**Zach L.** · 12-24-2003

You are certainly on the right path now. The virtual function with the derived classes will work just as you were wanting them to. I would however, change your other functions ('r( )' and 'b( )'). Since they store the transmission type and color, these are attributes of 'Vehicle', and could be implemented in a single function there:

Code:

// For a quick example...
class Vehicle
{
public:
  void info( ) { cout << "The color is " << m_color << endl; }
};

// ... etc ...

int main( )
{
  Suv v;
  Sportscar r;
  v.info( );
  r.info( );
}

That way, you'd store the transmission type and color in the base class. You may even want to make that function virtual, so you could reimplement it to output additional information in a future derived class if necessary.

Also, it would be advisable to make the 'Vehicle' destructor virtual to avoid nasty side-effects if you end up dealing with pointers to vehicles.

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